Optical communication systems presently carry large amounts of data. A fibre break or other interruption along a line can thus potentially affect multiple services. In order to restore these services with the minimum of delay, the network must rerout the traffic via an alternative path. This is accomplished in the SONET/SDH system by electrically switching and routing traffic signals in every node. When a fiber break or other disturbance is detected which prevents transmission over a particular link, the nodes can be reconfigured to switch traffic signals via alternative nodes.
However, for WDM systems and more generally for optical networking, where multiple protocols, such as IP, ATM, Gigabit Ethernet and the like, coexist with SONET/SDH systems, electrical switching in each node is not practicable.
Such networks are therefore typically configured with at least one alternative path formed by a separate direct optical link between the nodes in a network. The alternative path is commonly called a protection path. If information does not arrive via the first path or working path, transmission is switched to the alternative path. For total protection, complete redundancy is required, with the fill link including transmitters and receivers duplicated. However, such an arrangement is naturally very costly, particularly for WDM systems as multiple transmitters are required for each path. Moreover, for many applications the degree of protection assured by fill redundancy is not needed.
A reduction in cost is obtained if a single transmitter is used for both the working and protection paths. This may be achieved by providing a switch to control the connection between the two paths and the single transmitter. However in operation, information about a failure in one path must be obtained and relayed to the switch control.
The need for switching may be overcome by dividing the signal power from the transmitter equally between the two paths using a splitter. However the division of power in this way will impose a 3 dB loss on each path, in addition to the loss in the splitter itself. These additional losses reduce the possible transmission distance without amplification. Fiber losses are typically of the order of 0.25 dB/3 km. Imposing a power reduction of 3 dB on a path effectively shortens the possible link distance by around 12 km.